JP-A-2002-126390 and JP-A-2001-178992 disclose conventional techniques for detecting a weight of laundry put into a rotating tub of a washing machine, for example. In the disclosed techniques, a rotation sensor is mounted on an electric motor to detect a rise time required for the rotational speed of the motor to change from a first rotational speed to a second rotational speed while a predetermined power is being supplied to the motor. A weight of laundry is detected according to the detected rise time.
However, the above-described conventional techniques have the following problems. Firstly, in order that the predetermined power may be supplied to the motor, motor voltage is controlled to be held constant. However, output differs upon load variations even under the condition where motor voltage is constant. As a result, an accurate detection cannot be carried out.
Secondly, the above-described detection is equivalent to detecting acceleration of the motor and requires a long period of time equal to the aforesaid rise time. Furthermore, the foregoing first reason tends to increase variations in the results of detection. Accordingly, rise time detection needs to be carried out at a plurality of times. Thus, a long time is required for the rise time detection.
The present invention was made in view of the foregoing and an object of the present invention is to provide a washing machine in which the weight of laundry can be detected quickly and accurately.